Mech 466
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1. Changes the value of a control or selects the text within a control gt Positioning tool Positions resizes and selects objects gt Labeling tool Edits text and creates free labels gt Wiring tool Wires objects together in the block diagram gt Object Pop up Menu tool Opens the pop up menu of an object gt Scrolling tool Scrolls the window without using the scroll bars gt Breakpoint tool Sets breakpoints on VIs functions loops sequences and cases gt Probe tool Creates probes on wires gt Color Copy tool Copies colors for pasting with the Color tool gt Coloring tool Sets foreground and background colors Selection Positioning Teal beling Tool Operating Tool scrolling Tool Wiring Taal Probe Tool Breakpoint Toul Color Copy Tool wWoring Tool Figure 3 Tools Palette Mech 466 Automatic Controls Laboratory Manual Lab 4 Tank Level Display 62 3 2 Controls Palette The Controls palette consists of a graphical floating palette that automatically opens when you launch LabVIEW You use this palette to place controls and indicators on the front panel of a VI Each top level icon contains subpalettes If the Controls palette is not visible you can open the palette by selecting Windows Show Controls Palette from the front panel menu You can also pop up on an open area in the front panel to access a temporary copy of the Controls palette Figure 4 displays the top level of the Controls2. In this section you are asked to build the Tank Level Display VI whose front panel and block diagram are shown in Figures 1 and 2 The purpose of the VI is to perform the following functions gt To continuously acquire signals from two pressure transducers placed in the base of two tanks and display the corresponding readings 12 bit unsigned integer counts gt Toconvert the acquired counts into voltages and display them gt To convert voltages into liquid heights display them and monitor their time response Inlet Inlet Valve A Valve B Tank A Tank B Pressure Pressure Transducer A Transducer B j Process Disturbance v Z Process Valve B RA S aaa ee a OOM EEESEEEE 55 EE NE iE ggg X xggg VCVCVNEN An E sgggd Figure 7 Schematic of the Tank Level Display and Control Setup Mech 466 Automatic Controls Laboratory Manual Lab 4 Tank Level Display 70 Creating the front panel Begin by considering the front panel shown in Figure 1 It has three digital indicators for Volts A Volts B and Time sec two tank level indicators to display the current liquid levels a waveform chart that displays the time response of the tanks levels and a stop button to stop acquiring data Consider the following steps in building the front panel 1 Select New VI from the LabVIEW startup screen 2 Create the three digital indicators Select Digital Indicator from the Numeric Indicator
3. 5 volts 2048 for 0 volts and 4095 for 5 volts Table 1 shows the bit counts versus resolution and voltage value of one bit Multiplexing A multiplexer is a switch that allows a single A D converter to measure many input channels often 8 12 or 16 channels This switch is normally a solid state switch to allow for high speed switching between channels A multiplexed scheme eliminates the high cost of having multiple A D converters However multiplexing reduces the rate at which data can be acquired from an individual channel because multiple channels are scanned sequentially Mech 466 Automatic Controls Laboratory Manual Lab 4 Tank Level Display 67 Table 1 Resolution and Accuracy for A D Conversion ESE Value ae 2 5 V E Resolution Number al 10 Y 0 10 V 1 25 V 2 02 V __ bits __ Counts dB of Range Range 5 W Range Range Range a 256 48 2 39 o 78 mV 39 mV i JAMY AG mv i0 1 024 60 2 gags 20 mV 98 my 24 mv C4 mv 12 4 056 72 2 024 49 mv 24 my Bi mv 01 mv 16 65 536 96 3 1S ai mY AS my 4 mY 000k mi 20 148 576 174 000095 _ A 24 16 777 216 1445 000008 The software LabVIEW is designed with drivers that can control the operation of the DAQ card Key elements of the data acquistion such as sample rates number of input channels etc can be selected from software inputs It should be noted however that signal conditioning operations such as the transducer excitation voltages and amplification
4. Tank Level Display VI The front panel of a VI is primarily a combination of controls and indicators Controls simulate instrument input devices and supply data to the block diagram of the VI Indicators simulate instrument output devices that display data acquired or generated by the block diagram of the VI if Tank Level Display vi Front Panel Jax Fle Edit Operate Toots Browse Window Help l0 fn woke A Figure 1 Front Panel of a Tank Level Display VI You add controls and indicators to the front panel by selecting them from the floating Controls palette You can change the size shape and position of a control or indicator In addition each control or indicator has a pop up menu you can use to change various attributes or select different menu items You access this pop up menu by right clicking the object with the mouse button Mech 466 Automatic Controls Laboratory Manual Lab 4 Tank Level Display 59 2 2 Block Diagram You can switch from the front panel to the block diagram by selecting Windows Show Diagram from the menu The Diagram window holds the block diagram of the VI which is the graphical source code of a VI You construct the block diagram by wiring together objects that send or receive data perform specific functions and control the flow of execution Figure 2 shows the block diagram of the Tank Level Display VI The diagram shows several primary block diagram program objects terminals nodes and wir
5. are normally done outside of the computer in the signal conditioning box Review of the modeling of fluid flow through a constriction The general form for the relation of the mass flow rate w of a fluid flow through a constriction valve orifice etc and the pressure difference p p2 across this restriction is given by l a 1 fo gt a7 1 Here R is the resistance of the restriction and is a constant that takes on values between 1 and 2 depending on the type of restriction The most common value of in the case of high flow rates through pipes those having a Reynolds number Re gt 10 or through short constrictions or nozzles is 2 For very slow flows through long pipes where the flow remains laminar Re lt 1100 1 Flow rates between these two extremes can yield intermediate values of a Note that a value of 2 indicates that the flow is proportional to the square root of the pressure difference and therefore will produce a nonlinear equation For the initial stages of control systems analysis and design it is typically very useful to linearize these equations so that the well established design techniques for linear systems can be applied Linearization involves selecting an operating point and expanding the nonlinear term to be a small perturbation from that point The nonlinear term can be expanded according to the relation Ite 1 4 fe 2 where s lt lt 1 In the case of a liquid t
6. the control flow method of executing a conventional program in which instructions are executed in the sequence in which they are written Control flow execution is instruction driven Data flow execution is data driven or data dependent 2 3 Icon and Connector When an icon of a VI is placed on the diagram of another VI it becomes a subVI the G equivalent of a subroutine The controls and indicators of a subVI receive data from and return data to the diagram of the calling VI The connector is a set of terminals that correspond to the subVI controls and indicators The icon is either the pictorial representation of the purpose of the VI or a textual description of the VI or its terminals The connector is much like the parameter list of a function call the connector terminals act like parameters Each terminal corresponds to a particular control or indicator on the front panel A connector receives data at its input terminals and passes the data to the subVI code through the subVI controls or receives the results at its output terminals from the subVI indicators Every VI has a default icon displayed in the icon pane in the upper right corner of the front panel and block diagram windows Every VI also has a connector which you access by choosing Show Connector from the icon pane pop up menu on the front panel When you bring up the connector for the first time you see a connector pattern You can select a different pattern if you want The conne
7. you may need are the Helo Window and the Reference Help Both help options can be accessed from the Help pull down menu The Help Window contains help information for functions constants subVIs controls and indicators and dialog box menu items To display the window choose Show Context Help from the Help menu Move your cursor over the icon of a function a subVI node or a VI icon including the icon of the VI you have open shown at the top right of the VI window to see the help information The Help window displays the most commonly required help information in a condensed format To access more extensive online information select Helo Online Reference For most block diagram objects you can select Online Reference from the pop up menu of the object You also can access this information by pressing the question mark button shown to the left located at the bottom of the Help Window For a more extensive explanation of LabVIEW functionality refer to the LabVIEW User Manual or the G Programming Reference Manual 5 Data Acquisition Systems The use of LabVIEW software and computer form a portion of a modern data acquisition system A data acquisition system commonly called DAQ system is a term normally applied to a microcomputer based measurement sampling and processing system The basic form of a data acquisition system is illustrated in Figure 6 A DAQ system begins with the transducers in our case pressure transducers mounted on the b
8. Lab 4 Tank Level Display 57 Mech 466 Automatic Controls Lab 4 Introduction to Data Acquisition and LabVIEW Part 1 Displaying Water Levels in a Tank LEARNING OBJECTIVE The objectives of the experiments in this lab are gt Introduce the LabVIEW program development environment as an efficient tool for data acquisition and process control gt Introduce and apply data acquisition concepts and methodologies gt Build a LabVIEW program that acquires voltage signals corresponding to liquid heights gt Calibrate the LabVIEW program to display actual liquid heights gt Calculate the resistance of valves based on formulas that model fluid flow through restrictions and experimental data obtained using the developed LabVIEW program This Lab also serves as an introduction that covers the necessary background required for Lab 5 in which the actual control of liquid height is performed v BACKGROUND 1 What Is LabVIEW LabVIEW is short for Laboratory Virtual Instrument Engineering Workbench It is a program development environment much like modern C or BASIC However LabVIEW is different from those development environments in one important respect While other programming systems use text based languages to create lines of code LabVIEW uses a graphical programming language G to create programs in block diagram form 2 How Does LabVIEW Work LabVIEW is a general purpose programming system but it also includes libraries of
9. am may allow the VI to perform a single data acquisition operation for the tanks level To perform this operation repeatedly the objects in the block diagram must be placed inside a While Loop found in the Structures subpalette of the Functions palette The While Loop is a resizable box You place the While Loop in the block diagram by clicking with the While Loop icon in an area above and to the left of all the objects that you want to execute within the loop then drag out a rectangle while holding down the mouse button The While Loop has two terminals a Boolean input terminal if symbol is loop executes while input is Mech 466 Automatic Controls Laboratory Manual 11 12 13 14 Lab 4 Tank Level Display 72 TRUE and an optional numeric output terminal that outputs the number of times the loop has executed NOTE In the diagram the constants for the device and for the channels are placed outside of the while loop Having these constants outside of the loop has no bearing on how the program operates but if you were to want those to be controls so that you could configure which channels are which only on program start then it is vital that the controls be outside of the while loop otherwise during each loop the program would recheck these controls and set them to what ever the values are at the time of activation The stop button HAS to be in the while loop otherwise it only checks the status of the button once at startup and does
10. ank with a liquid height h area A and no inlet flow w 0 and a relatively short restriction at the outlet with a 2 the continuity equation gives m pAh SW WV ie WY at 3 Mech 466 Automatic Controls Laboratory Manual Lab 4 Tank Level Display 68 The pressure at the entrance of the restriction is given by p pgh and assuming ambient pressure outside the restriction p2 0 the equation describing the outflow through the restriction can be written as L S Wout pgh Combining 3 and 4 gives pAh y pgh 5 Linearization of 5 involves selecting an operating point h and substituting h h Ah and h Ah At to obtain pa path Ah At ogh ah R oO Since lt lt 1 therefore using 2 oO At R Ah Vp8h Crd 2h i 6 Thus the resistance of the restriction can be obtained as ERA TE A p 2 h J Ah 7 Note that since the height is decreasing Ah will be negative and R will be a positive value Mech 466 Automatic Controls Laboratory Manual Lab 4 Tank Level Display 69 PROCEDURES Tank Level Display This Lab demonstrates how LabVIEW can be used for process monitoring and control applications The actual control however is performed in Lab 5 Figure 7 shows a schematic for an experimental setup that resembles many real life applications It consists of two interconnected tanks that continuously supply two different processes with two different flow rates
11. ch top level icon contains subpalettes If the Functions palette is not visible you can select Windows Show Functions Palette from the block diagram menu to display it Then you can click the All Functions button to bring up the All Functions Palette You can also pop up on an open area in the block diagram to access a temporary copy of the Functions palette and access the same palette by clicking on the All Functions button there Figure 5 displays the all Functions level of the Functions palette And a brief description of the buttons is on the flowing page Numeric Structures Nap A search Sz Bacless lime and Dialog Clusters Strings Waveform Ci Mparis mn File I 00 WNI and Instrument i Analyze i i Application Control Report Generation Communication Advanced g mn a i T I elect a V1 User Libraries Decorations Figure 5 All Functions Palette Mech 466 Automatic Controls Laboratory Manual Lab 4 Tank Level Display 64 gt Structures Contains Sequence Structures Case Structures For Loops While Loops and Formula Nodes The icon for each G structure is a resizeable box with a distinctive border You can place an object inside a structure by dragging it inside or by building the structure around the object The Sequence Structure consists of one or more subdiagrams that execute sequentially The Case structure executes a subdiagram based on an input value and is analogous to an if then else s
12. ctor generally has one terminal for each control or indicator on the front panel You can assign up to 28 terminals If you anticipate future changes to the VI that might require a new input or output leave some extra terminals unconnected 3 What Are LabVIEW Palettes LabVIEW palettes contain various tools and objects used to create and edit the front panel and block diagram It is said that the palettes float because you can move them anywhere you want on the desktop The three main palettes are the Jools Controls and Functions palettes Mech 466 Automatic Controls Laboratory Manual Lab 4 Tank Level Display 61 3 1 Tools Palette LabVIEW uses a floating Tools palette which you can use to edit and debug VIs A tool is a special operating mode of the mouse cursor You use tools to define the operation taking place when subsequent mouse clicks are made on the panel or diagram You can move this palette anywhere you want or you can close it temporarily by clicking the close box Once closed you can access it again by selecting Windows Show Tools Palette Figure 3 displays the Tools palette with the following functions gt Automatic Tool Selection lIf automatic tool selection is enabled and you move the cursor over objects on the front panel or block diagram LabVIEW automatically selects the corresponding tool from the Tools palette You can disable automatic tool selection and select a tool manually gt Operating tool
13. easuring by selecting the N Measurement then selecting the Data Acquisition subpalettes from the All Functions palette 4 Right click on one of the channels and select New Channel Make sure the drop down list box has Analog Input selected then press the next button Type Tank A for channel name and leave the description blank then press the next button if Tank A is already made create a Tank C for practice Make sure that Voltage is selected on the drop down menu then press the next button this ensures that the readings we are getting are Voltage measurements Now it is asking you to name the units for this measurement this is optional but put Volts in the Units box and make sure the minimum and maximum values are set to 5 Volts and 5 Volts respectively then press the next button Make sure that No scaling is selected then again press the next button It then asks what device you want to use this allows you to select between multiple devices on systems that have more than one Select Dev1 and set Which channel to 0 and select the analog input mode of Referenced single ended then click the finish button Repeat for Tank B but make the analog channel Which channel set to 1 5 Now left click on the first channel and set it to Tank A and left click on the second channel and set it to Tank B Note that you may select Tank A or Tank B on both channel tools of the block d
14. es When you place a control or indicator on the front panel a corresponding terminal appears on the block diagram You cannot delete a terminal that belongs to a control or indicator The terminal disappears only when you delete its control or indicator on the front panel The terminal symbols suggest the data type of the control or indicator For example a DBL terminal represents a double precision floating point number a TF terminal is a Boolean an 116 terminal represents a regular 16 bit integer and an ABC terminal represents a string The Multiply function icon also has terminals Think of terminals as entry and exit ports Terminals that produce data are referred to as data source terminals and terminals that receive data are data sink terminals ifs Tank Level Display vi Block Diagram File Edit Operate Tools Browse Window Help TE G alo ir askin Fe E Slope Intercept gt gt ank Level Disp la Tank 5 Tank B 7 le o Figure 2 Block Diagram of the Tank Level Display VI Nodes are program execution elements They are analogous to statements operators functions and subroutines in conventional programming languages The Multiply function is one type of node The G programming language has an extensive library of functions for math comparison conversion I O and more Another type of node is a structure Structures are graphical representations of the loops and case statements of traditional progra
15. functions and development tools designed specifically for data acquisition and instrument control LabVIEW programs are called virtual instruments VIs because their appearance and operation can imitate actual instruments However VIs are similar to the functions of conventional language programs A VI consists of an interactive user interface a data flow diagram that serves as the source code and icon connections that allow the VI to be called from higher level Vis More specifically Vis are structured as follows gt The interactive user interface of a VI is called the front panel because it simulates the panel of a physical instrument The front panel can contain knobs push buttons graphs and other controls and indicators Mech 466 Automatic Controls Laboratory Manual Lab 4 Tank Level Display 58 gt The VI receives instructions from a block diagram which you construct in G The block diagram is a pictorial solution to a programming problem The block diagram is also the source code for the VI gt Vis are hierarchical and modular You can use them as top level programs or as subprograms within other programs A VI within another VI is called a subVI The icon and connector of a VI work like a graphical input output parameter list so that other VIs can pass data to a subVI 2 1 Front Panel The user interface of a VI is like the user interface of a physical instrument the front panel Figure 1 shows the front panel of a
16. ght in Tank B The above graphs are based on the assumption of slow laminar flow through the valve However as indicated in the review section a value of 2 is more suitable for high flow rates and flows through short constrictions such as valves This yields the nonlinear relation in Equation 5 s d pah NN which can be linearized by selecting an operating point h to give rp _t gh 14h At A p 2 h J Ah Assuming an operating height for the water in tank A to be hao 10 inches use the time interval that contains this height to find the resistance of Process Valve A based on the Mech 466 Automatic Controls Laboratory Manual Lab 4 Tank Level Display 74 linearized equation at this operating point Also find a linearized value for the resistance of Process Valve B at an operating height of hg 7 inches To find the resistance of Valve AB at the same operating point hy 10 inches and hg 7 inches fill Tank A to an initial height of 12 inches and Tank B to 5 inches Open Valve AB and allow Tank A to drain into Tank B Record the time it takes for Tank A height to drop from 10 5 inches to 9 5 inches or Tank B height to rise from 6 5 inches to 7 5 inches In this case assuming the water height in Tank A is higher than that in B the flow through Valve AB that connects the two tanks A and B can be written as Ah Ah 1 pAr eee yp8 h h z At At R which can be linearized about the operating poi
17. iagram even though you only setup each once This is so that users can just setup the channels once at the beginning and have them available anywhere in the program 6 Place two Multiply functions and two Add functions on the block diagram These are located in the Numeric subpalette of the Functions palette All these functions have two input terminals and one output terminal 7 Add the two sets of Volts to Inches linear conversion factors with preliminary values of 1 0 1 0 0 0 and 0 0 to the block diagram by selecting Numeric Constant from the Numeric subpalette of the All Functions palette Label these constants as slope and intercept as shown The values of these conversion factors will be later adjusted 8 The waveform chart has a single input terminal For the waveform chart to plot two variables the two variables must be bundeled together into a cluster using the Bundle function available in the Cluster subpalette of the All Functions palette Add a Bundle function to the block diagram The Bundle function has two input terminals and one output terminal 9 Using the wiring tool found on the Tools palette wire the terminals of the objects on the block diagram as shown in Figure 2 To wire from one terminal to another click on the first terminal move the tool to the second terminal and click on the second terminal Remember that it does not matter on which terminal you initiate the wiring 10 The current status of the block diagr
18. mming languages The G programming language also has special nodes for linking to external text based code and for evaluating text based Mech 466 Automatic Controls Laboratory Manual Lab 4 Tank Level Display 60 formulas You add nodes to the block diagram by selecting them from the floating Functions palette Wires are the data paths between source and sink terminals You cannot wire a source terminal to another source nor can you wire a sink terminal to another sink However you can wire one source to several sinks Wires are colored according to the kind of data each wire carries Blue wires carry integers orange wires carry floating point numbers green wires carry Booleans and pink wires carry strings To wire from one terminal to another select the Wiring tool from the floating Tools palette click on the first terminal move the tool to the second terminal and click on the second terminal It does not matter at which terminal you start The hot spot of the Wiring tool is the tip of the unwound wiring segment When the Wiring tool is over a terminal the terminal area blinks to indicate that clicking connects the wire to that terminal Data flow is the principle that governs G program execution Stated simply a node only executes when all data inputs arrive the node supplies data to all of its output terminals when it finishes executing the data immediately passes from source to sink terminals Data flow programming contrasts with
19. not change the value during execution of the program as it is not asked to As it stands now the While Loop would execute as quickly as the computer system would allow To control the loop timing place the Wait Until Next ms Multiple function located in the Time amp Dialog subpalette of the Functions palette inside the loop This timing control function has a single input terminal and a single optional output terminal It waits until the millisecond timer is a multiple of the specified input value in ms before returning the optional millisecond timer value Wire a Numeric Constant of 100 0 ms to the input terminal of the timing function To display the elapsed time since the start of the run first convert the time step of the While Loop execution in milliseconds to seconds by dividing by a 1000 0 then multiply it by the number of times the loop has executed i and finally wire the result to the Time sec digital indicator To stop acquiring data and exit the While loop the stop button terminal should be wired to the Boolean input terminal of the loop Since the default output value of the stop button is FALSE and clicking the button will change it into TRUE the while loop by default stops on a TRUE condition thus wiring the button direct to the Boolean input terminal will stop the program when the stop button is pressed Select Save from the File menu to save your work on a floppy disk You can run the VI by clicking on the Run button
20. nt ha Nao AM and hg hg Ahs to give h h 1 Ah Ah A ieee 8 Ny Mp es B j A P 2 hy ae Ah Report Requirements 1 Have the TA inspect the developed VI and its operation Include the TA s comments on the VI 2 Comment on any difficulties you encountered in building the VI 3 Comment on the usefulness and applicability of G programming using LabVIEW 4 Graph the calibration results Find the slope and intercept for both graphs Comment on the results including any sources of errors 5 Calculate and graph the valve resistance versus tank heights for both Process Valves A and B based on the linear assumption Is this assumption valid Explain 6 Using the linearized equations determine the resistance of the three valves at the operational points Compare these values with the corresponding linear case Comment on the units of the resistance in each case Which value of R should be used in the process control of the tanks Explain Mech 466 Automatic Controls Laboratory Manual
21. on either the front panel or block diagram Toolbars Calibrating the VI to find the volts to inches conversion factors If you run the VI as its stands now you will find that the displayed tanks levels are the voltage readings and not the actual heights Selecting appropriate values for the volts to inches linear conversion factors slope and intercept for each of the two pressure transducers can be accomplished as follows l 2 oe Run the Tank Level Display VI While Inlet Valve A open and all other valves closed turn on the pump to fill Tank A with water to a known height say 6 inches Turn the pump off wait a second then record the corresponding voltage Turning the pump off also reduces the noise in the voltage reading Repeat step 2 for water heights of 8 10 12 20 inches Use the eight data points to plot the height against the voltage for Tank A The slope and intercept can then be measured graphically Close Inlet Valve A open Inlet valve B and repeat steps 2 4 for Tank B Stop the Tank Level Display VI Insert the slope and intercept values into the block diagram by typing them over the 1 0 1 0 0 0 and 0 0 preliminary values for the conversion factors Now the Tank Level Display VI is ready to display actual tanks levels Mech 466 Automatic Controls Laboratory Manual Lab 4 Tank Level Display 73 Finding the resistances of Process Valve A Process Valve B and Valve AB In this section you are
22. ottom of the tanks that outputs a voltage or current signal in response to the variable of interest water pressure A signal conditioning system contains electronic equipment designed to supply the source voltage to the transducer as well as provide filtering and amplification The computer contains a DAQ card that provides the necessary analogue to digital conversion and or digital to analogue and using DAQ software will perform all data manipulation data storage and display Data Acquisition System Components Signal Conditioning Analogue signals will normally require some type of signal conditioning for the proper interface with a digital system These components include 1 Transducer Excitation Certain transducers such as strain gauges require external voltages or currents to excite their own circuitry in a process known as transducer excitation It is important that the supply voltage provided by the signal conditioner be stable and relatively noise free 2 Amplification Amplification maximizes the use of the available voltage range to increase the accuracy of the digital signal and to increase the signal to noise ratio SNR Although computer DAQ boards will often provide some selectable signal amplification it should be remembered that external amplification is often necessary to minimize the effects of external noise 60 Hz noise for example Our system Mech 466 Automatic Controls Laboratory Manual Lab 4 Tank Level Dis
23. palette gt Vv VY WV WV VV VY Num Ctrls Contains numeric controls for entering numeric quantities Includes digital controls such as knobs and dials Buttons Contains Boolean controls that simulate switches buttons A Boolean control or indicator has two values TRUE or FALSE Text Ctrls Contains string controls and indicators for entering and displaying text User Cirls Contains any user defined controls that the user wishes to have readily available Num Ctrls Contains numeric indicators for dispaying numeric quantities Includes digital indicators such as meters thermometers and tank level indicators LEDs Contains Boolean indicators that simulate LEDs Text Inds Contains string and path indicators as well as tables A string is a collection of characters used to make up words or sentences Graph Inds Contains graph and chart indicators for data plotting All Controls Contains all of the controls above plus many others for more complex designs Text Ctrls b obuc Num Inds i Text Inds Graph Inds Figure 4 Controls Palette Mech 466 Automatic Controls Laboratory Manual Lab 4 Tank Level Display 63 3 3 Functions Palette The Functions palette consists of a graphical floating palette that automatically opens when you switch to the block diagram You use this palette to place nodes constants indicators Vis and so on on the block diagram of a VI Ea
24. play 66 utilizes a dedicated strain guage card specially designed for wheatstone circuit based instruments such the pressure transducer 3 Filtering Filters are required to remove unwanted signals from the desired signal Several types of filters such as low pass high pass and band pass filters are installed in the signal conditioning to clean the signal prior to digital processing The filtering card in this experiment is set with a cut off or corner frequency of 100 Hertz Signals voltage current ete Fhysical phenomena temperature pressure flow displacement light intensity density ete Figure 6 Components of a Data Acquisition System Data Acquisition Card The data acquisition card located within the computer contains the circuitry to provide the necessary Analogue to Digital conversion for signal processing Some of the key features of a DAQ card are as follows A D Converters The A D converter converts the input analogue voltage to a digital signal which can be read and processed by the computer The resolution and accuracy of the conversion depends on the number of bits the voltage is translated into The conversion time varies as the resolution i e the more bits required the longer it takes i e 10 usec for 12 bit conversion versus 20usec for 16 bit conversion The DAQ card installed in the ME466 computers IOTech DAQboard 100 utilizes a 12 bit A D converter It is configured to provide a 0 count for
25. required to perform the experimental measurements needed to find the resistances of three valves Referring to the review section that covers the modeling of fluid flow through restrictions it can be seen that the formula for the resistance of the outlet flow valve of a tank with no inlet flow depends on the flow speed For very slow flows the constant in Equation 1 is assumed to equal 1 This yields a linear flow rate relation involving the valve resistance R in the form Ah pgh p 2 8 and assuming h Ah At we get gh At A Ah Knowing that the inside diameter of both tanks is 5 inches and the outside diameters of the two pipes that run inside each tank are 1 25 inches and 0 875 inches use the following steps to find the resistance of Process Valves A and B 1 Run the Tank Level Display VI 2 While Inlet Valve A open and all other valves closed turn on the pump to fill Tank A with water Turn the pump off 3 Open Process Valve A and let the tank drain 4 Using the time indicator on the front panel of the Tank Level Display VI monitor the height drop from about 20 inches to about 5 inches by recording the time and the corresponding tank height using 10 seconds time intervals 5 Use the linear equation for the flow rate through a constriction 1 to graph the resistance of Process Valve A versus the water height in Tank A 6 Use a similar procedure to graph the resistance of Process Valve B versus the water hei
26. s Num Inds subpalette of the Controls palette Drag and drop the indicator on the front panel Type Volts A inside the label box Using the positioning tool from the Tools palette drag the indicator to the desired location on the front panel Repeat the same process to create the other two indicators and label them Note that each time you create a new control or indicator LabVIEW automatically creates the corresponding terminals in the block diagram 3 Similarly create the two tank level indicators by dragging them from the Numeric subpalette of the Controls palette Add labels and position the tanks You can set the upper limit of the scale of each tank to read 25 0 inches by direct typing over the initial 10 0 upper limit 4 Create the waveform chart by selecting it from the Graph Indicators Graph Inds subpalette of the Controls palette Add the chart label position it and set its y axis upper limit If you right click on the graph using the mouse a pop menu will appear that can allow you to control the functionality and appearance of the chart such as showing or hiding the graph palette and the x axis scale Set the x axis scale to AutoScale X also select the y axis scale and make sure that there is no check mark beside AutoScale Y You can also resize the chart s legend by dragging its corners to allow for the two variables 5 Finally create the stop button by selecting it from the Boolean subpalette of the Controls palette Crea
27. sis Contains functions to perform waveform measurements waveform conditioning waveform monitoring waveform generation signal processing point by point analysis and mathematical calculations VV VV VVNMN WV Application Control Contains functions to programmatically control Vis and LabVIEW applications on the local computer or across a network Communication Contains functions to exchange data between applications Report Generation Contains functions to create and manipulate reports of LabVIEW applications Advanced Contains functions to call code from libraries such as dynamic link libraries DLLs to manipulate LabVIEW data for use in other applications and to call code from text based programming languages gt Decorations Contains items to group or separate objects on a block diagram with boxes lines or arrows These objects are for decoration only and do not modify data gt User Libraries Use the User Libraries palette to add controls and VIs to the Controls and Functions palettes By default the User Libraries palette does not contain any objects Vv WV WV WV Mech 466 Automatic Controls Laboratory Manual Lab 4 Tank Level Display 65 gt Select a V For selecting VIs to be inserted as subVIs in the current VI The icon and connector pair of a VI must be defined before it could be used as a subVI in another VI 4 How To Get Help In LabVIEW The two common help options that
28. tatement in text based programming languages You use the For Loop and While Loop to control repetitive operations either until a specified number of iterations completes For Loop or while a specified condition is true While Loop The Formula Node executes one or more mathematical formulas gt Numeric Contains functions that perform elementary operations on numbers like add and multiply It also contains numeric constants for use in the block diagram Boolean Contains functions that perform elementary operations on Booleans It also contains Boolean constants String Contains functions that work on strings and string formatting Array Contains functions that work on arrays Cluster Contains functions that work on clusters such as Bundle and Unbundle Comparison Contains functions that compare numbers Time amp Dialog Contains functions that work as timers and functions that display dialog boxes File I O Contains functions that read write input output files NI and Instrument I O Contains functions that communicate with input output instruments there is a NI Measurements button which is more specific to National Instruments products that also contains instrument input output as well gt Waveform Contains functions to build waveforms that include the waveform values channel information and timing information and to set and retrieve waveform attributes and components gt Analy
29. ting the block diagram Switch to the block diagram window and note the different terminals that LabVIEW automatically created corresponding to the front panel objects Use Figure 2 as a guide for positioning the different terminals in the diagram The subVI Get Single Scan vi is set up to communicate with the data acquisition card installed in the computer and to perform a single scan on two specified channels to which the pressure transducers are connected The Get Single Scan vi SubVI takes the device number and the channels to scan as input The SubVI also returns two double values corresponding to the read pressure signals The output from Two Ch Single Scan vi subVI are 12 bit unsigned integers ranging from zero to 4095 largest 12 bit integer that are internally converted to a double value in Volts The voltages are then converted into liquid heights using two sets of linear conversion factors each consisting of a slope and an intercept Use the following steps to build the block diagram Mech 466 Automatic Controls Laboratory Manual Lab 4 Tank Level Display 71 1 Insert the subVI Get Single Scan vi into the block diagram by choosing Select a VI from the Functions palette This subVI has two output terminals and a two input terminals 2 Create a new constant for the device input terminal and set the value to 1 by selecting the Numeric Constant from the Numeric subpalette of the All Functions palette 3 Create two channels for m
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